It has somewhat recently been determined that all high-pressure hydraulic systems, using phosphate base hydraulic fluids, whether installed in airplanes, land vehicles, steam turbine controls, or elsewhere, have the metering edges of metering passageways of their valves being subjected to metal removal, because of severe electrokinetic induced corrosion erosion. When this electrokinetic induced corrosion erosion occurs on the control or metering edges of metallic components of a hydraulic valve or other metering devices in a hydraulic system, high leakage flows soon result, which often lead to the loss of control of the valve flow gain and pressure gain, the instability of the mechanism under control, and the destructive overheating of the entire hydraulic system.
Efforts to reduce the hazards of fire, led to the use of these phosphate base fluids in commercial airplanes, starting in 1958, and steam turbine control systems, in the early 1960's. As a consequence in reference to airplanes, the personnel of British European Airways of England in 1965 began experiencing increased internal leakage, when operating the new British manufactured Trident airplanes, which employed powered controls. It was found that extreme erosion was taking place at the metering edges of the servo valves and other hydraulic valves.
Scientists, both chemists and physicists, who were experts in fluid flow, and also hydraulic engineers, from both Great Britain and the United States, all concluded at that time in 1965, or thereafter that this erosion was due to cavitation. For example, in a technical paper by W. Hamilton of the A.F.R.Ae. S. and the Technical Service Dept. of Hawker Sidley Aviation Ltd., published in the British periodical, Aircraft Engineering, December 1966, the problems, efforts and findings were described. Water was added to the phosphate based hydraulic fluids to cope with the extreme erosion problem. However this addition of water only reduced erosion in the presence of non contaminated hydraulic fluids. When the hydraulic fluids became contaminated with chemical halides, chlorine and fluorine, the addition of water no longer reduced the erosion.
In 1967 the same problems were experienced in American manufactured steam turbine control systems using phosphate base hydraulic fluids. It was found that periodic filtering of these fluids through Fullers Earth filters reduced the erosion to a rate that has been accepted, though it is not considered desirable.
Also in 1967 American manufactured airplanes, using phosphate base hydraulic fluids, were found to have developed the same erosion patterns. These erosion patterns and resulting problems are still continuing on today.
In 1969, scientists of The Boeing Co. in Seattle, Wash. determined the erosion patterns or problems were not caused by cavitation, but instead they were caused by electrochemical corrosion, now referred to as electrokinetic induced corrosion erosion. Their findings are set forth in the Boeing Scientific Research Laboratory, report D1-82-0839, entitled Corrosion of Servo Valves by Electrokinetic Streaming Current, written by J. Olsen, T. R. Beck, and D. W. Mahaffey and issued in September 1969.
Moreover, it has been determined that the severe electro-kinetic induced corrosion erosion takes place when the hydraulic valves are nearly shut off in the null position and the fluid flow is small and at a high velocity. In addition, it was noted that the phenomena of the electrokinetic induced corrosion erosion does not take place at all flows. At flows of hydraulic fluid beyond the null or close to null position, and on up to full flow, substantially no electrokinetic corrosion erosion takes place.
Since 1965, for over ten years, the efforts of engineers and scientists of more than 25 companies in the United States, Europe, and Japan have been involved in attempting to find a solution or solutions to reduce or eliminate the electrokinetic corrosion erosion taking place in the metering passageways of hydraulic valves and other metering devices controlling the flow of phosphate base hydraulic fluids. During this time and right up to the present moment, the specification and designs for hydraulic valves and other metering devices specify and show square or rectangular, sharp cornered, sharp edge, orifices in the sleeves of the hydraulic valves, and like sharp edges of slides of these hydraulic valves, that relatively move to increase or decrease the controlled opening of the orifice or metering passageway. To quote from directions set forth on the detailed drawings of at least two companies in reference to the fabrication of the metering passageway structures, it is said that the metering edges must be sharp and free of burrs.
The designer, while thinking about this electrokinetic corrosion erosion problem, must still provide a servo valve that has good response. The valve cannot under most conditions have a flat spot or dead zone at its neutral position, because the valve, when operated, must react quickly to changes. The square or rectangular orifice of servo valves provides a lineal change of flow as the valve opens. The sharp edge of the orifice in the fixed sleeve, and the sharp edge of the mating slide have provided in the past and up to date, the most satisfactory combination of shapes for the machinist to produce, machine, and trim to provide the flow and the change of flow desired. To obtain the desired results the metering edges of the slide must mate with metering edges of the ports in the sleeve within 1 to 3 10,000s of an inch.
Most design work on metering passages of hydraulic valves has been performed in the past to improve a valve's performance under high flow conditions and not under low quantity flow conditions in and near the null position of hydraulic valves. The valve shapes have been adopted to reduce pressure drops under high flow conditions and not to improve low quantity flow conditions at high velocities in and near the null position of hydraulic valves.
Today, the severe results continue to occur because of electrokinetic induced corrosion erosion of the valve components. In extreme breakdowns, the entire hydraulic system fails, when the maximum hydraulic pump capacity is needed to maintain the leakage flow, thus leaving no capacity for hydraulic control. Yet phosphate ester hydraulic fluids will continue to be used to reduce the hazards of potential fire and to reduce the dangers of those fires that do get started, especially in reference to airplanes. Consequently, this invention has been directed to the successful provision of metallic hydraulic valve components and methods of their manufacture to create overall hydraulic systems in which electrokinetic induced corrosion erosion failures are avoided. The success being realized centers on providing improved metallic hydraulic valve components to especially control the hydraulic flow, thereby avoiding severe electrokinetic streaming current and its generation of wall current which induces corrosion erosion.